1. Field of the Invention
The present invention relates to a dimmer circuit. More particularly, the present invention relates to a dimmer circuit of a light-emitting diode (LED) and an isolated voltage generator and a dimmer method thereof.
2. Description of Related Art
Light emitting diodes (LEDs) have advantages of small size, power-saving and high durability, and as fabrication processes thereof become mature, price of the LEDs decreases. Therefore, it is popular to use the LEDs as light source products. Moreover, since the LED has features of low-operating voltage (only 1.5-3V), initiative light-emitting, and having a certain brightness, wherein the brightness can be adjusted by voltage or current, and has features of impact resistance, anti-vibration and long lifespan (100,000 hours), the LED is widely used to various terminal equipments, such as vehicle headlamps, traffic lights, text displays, billboards and large screen video displays, and domains such as general level architectural lighting and liquid crystal display (LCD) backlight, etc.
FIG. 1A is a system schematic diagram illustrating a conventional dimmer circuit of an LED. Referring to FIG. 1A, the dimmer circuit 100 is a basic circuit that a buck constant current control chip LM3445 is applied for dimming an LED, and a technical manual of the chip LM3445 can be referred for a detailed circuit operation of the dimmer circuit 100. In the dimmer circuit 100, an alternating current (AC) signal VAC is first modulated by a triac dimmer according to a dimmer phase angle thereof, and then a pulse width detection circuit 110 fetches a modulation signal Vac modulated by the triac dimmer. Then, a low-pass filter circuit 120 converts a pulse width of the modulation signal Vac into a direct current (DC) voltage. The chip LM3445 controls a switch signal of a transistor 130 according to the DC voltage, so as to control a current magnitude of a load current ILED used for driving the LED.
FIG. 1B is a waveform diagram of the modulation signal and the load current of FIG. 1A. Referring to FIG. 1A and FIG. 1B, when the dimmer angle of the triac dimmer is increased, the pulse width of the modulation signal Vac is relatively narrowed. When the pulse width of the modulation signal Vac is narrowed, the DC voltage received by the chip LM3445 is decreased. Now, the chip LM3445 controls the transistor 130 to decrease the current magnitude of the load current ILED, and the brightness of the LED is darkened as the current magnitude of the load current ILED is decreased.
According to the circuit of FIG. 1A, ground points of the dimmer circuit 100 are all the same, i.e. the dimmer circuit 100 is an un-isolated dimmer circuit. Moreover, a dimmer method of the dimmer circuit 100 is to adjust the current magnitude of the load current ILED.